The advent of evolutional molecular engineering has made it feasible to create an enzyme (protein) forming the basis of life reaction or a gene coding therefor in laboratories. By this technology, an enzyme (protein) with new activity not occurring in nature can be produced and expected for use in various applications to the fields of medicine and engineering.
An enzyme (protein) or a gene coding therefor is composed of a polymer of amino acids or nucleotides as a block unit. In evolutional molecular engineering, a molecule with desired activity is selected from a pool of polymers consisting of random amino acid or nucleotide block units.
However, even if it is attempted to prepare polymers with every combination, there is a limit to the physical amount of compounds which can be synthesized, so there is a limit to the number of blocks which can be linked, and as a consequence, a too large protein or gene cannot be created. Further, in consideration of an in vitro evolutional system for translating a protein from a nucleic acid polymer, the appearance of "termination codon" terminating the translation is a great problem. Therefore, a microgene which is large to a certain extent is preferably used as a block unit to form a gene coding for a large protein.
There is the hypothesis that a large gene was born by repeatedly polymerizing a small gene (microgene) (S. Ohno & J. T. Epplen, Proc. Natl. Acad. Sci. U.S.A. 80:3391-3395). Because it is considered that a polypeptide rich in a simple repeating structure can easily have a stable secondary structure, evolutional molecular engineering directed at large proteins or genes requires the techniques of repeatedly polymerizing a short structural unit to synthesize a macromolecule (Nature 367:323-324, 1994).
At present, a rolling circle synthesis method is reported as a method of preparing a polymer consisting of a short repeating DNA unit (PNAS 92:4641-4645, 1995).
However, this method should go through a plurality of steps including phosphorylation reaction, linkage reaction, polymerization reaction, double-stranded chain forming reaction, so its complicated reaction system is problematic.
Under these circumstances, there have been demands for developments in a reaction system in which a gene polymer can be formed more simply.